Doom is Ultron without any explanation or development and no robot army.
Destroy the world just cuz he's angry with the military? Something humans did? They would've been better off showing that Doom just went insane being alone for year plus that green goop rotting his brain. I would've hated that too but at least it would've been clear.
At least Ultron clearly stated why he was trying to wipe out humanity going off Stark's mission statement of protect Earth.